It’s the closest any of a chilling new wave of young Somali-born radicals has ever come to a US terror strike, authorities say.

Terrorist-in-training Mohamed Osman Mohamud meticulously plotted “a spectacular show” of terrorism at a crowded Christmas tree-lighting ceremony in Oregon, brushing off concerns that women and children would be caught in the carnage, authorities said.

The 19-year-old radical — a naturalized US citizen, engineering student and reported rapper — was busted Friday as he tried to use a cellphone to blow up a van full of dummy explosives planted by the FBI in an elaborate sting operation.

Mohamud, a fitness freak who wrote for Jihad Recollections, including an article on Pilates for terror thugs, had already videotaped a testimonial to jihad — dressed up “Sheik Osama style” in a white robe and camouflage jacket.

On Friday, after declaring the phony bomb “beautiful,” the hapless terrorist tried twice to detonate the device.

“Mohamud appeared so eager, that he started to read and dial the number off the paper [the informant] was holding faster than [the informant] could recite it,” an FBI affidavit said.

The arrest underscored American officials’ concern about the radicalization of young Somali-born men living in the United States — and the training some of them may be seeking overseas for jihad.

A year ago, 20 Somali youth in one area of Minnesota — the new Lost Boys of Somalia — went missing and were presumed to have gone overseas.

Yesterday, officials insisted Mohamud wasn’t being directed by any foreign terrorist organization as he planned the horrific details of his holiday attack.

“They have a Christmas lighting and some 25,000 people that come” who should be “attacked in their own element with their families celebrating the holidays,” he said, quoting Osama bin Laden, documents report.

“I want whoever is attending that event to leave, to leave dead or injured.”

The plot began to unravel last year when the feds, tipped off by the Muslim community, monitored Mohamud’s e-mail and found he was in contact with people overseas, asking how he could travel to Pakistan and join the fight for jihad, the affidavit showed.

On June 23, an undercover agent contacted Mohamud by e-mail, pretending to be affiliated with the “unindicted associate” to whom Mohamud had sent e-mails.

As the suspected young terrorist laid out his plot, he was warned several times about the seriousness of his plan, that women and children could be killed, and that he could back out.

Instead, he told undercover agents: “Since I was 15, I thought about all this” and “It’s gonna be a fireworks show . . . a spectacular show,” according to the affidavit.

The feds then provided Mohamud with a dud bomb that failed to go off when the terrorist wannabe used a cellphone detonator at a Christmas ceremony in Portland’s Pioneer Courthouse Square.

“The threat was very real,” said Arthur Balizan, special agent in charge of the FBI in Oregon. “Our investigation shows that Mohamud was absolutely committed to carrying out an attack on a very grand scale.”

When agents busted him, Mohamud struggled, kicking and screaming “Allahu Akbar,” Arabic for “God is great.” The Oregon State University student could face life in prison on a charge of attempting to use a weapon of mass destruction.